GB Pat. No. 423093 to Marguerre (1933) disclosed the basic principle of a heat pump and a heat engine for storing mechanical energy as heat in liquid storage media and using a reversible Rankine cycle.
U.S. Pat. No. 8,443,605 to Sapiem and U.S. Pat. No. 8,656,712 to Isotropic Ltd. store heat in a hot and a cold gradient storage with solid material using a Brayton cycle. EP Pat. No. 2778406 to ABB Technology AG stores energy in liquids using a Brayton cycle. WO 2010/145963 by ABB Research Ltd. uses a trilateral cycle and stores energy in liquids.
U.S. Pat. Appl. Publ. No. 2014/0060051 to ABB Research Ltd. discloses use of a zeotropic mixture and four liquid tanks. U.S. Pat. Appl. Publ. No. 2014/0245756 to Highview discloses use of separate liquid air and solid thermal storage media, using both latent heat from the liquid air and sensible heat from the solid media.
The disadvantage with Brayton cycles is low combined efficiency, with the working fluid flowing through two gas compression and two gas expansion processes in a complete charge and discharge cycle. The disadvantage of reversible Rankine cycles is the second law losses from the superheat in charging mode and the gradient preheating of the working fluid in discharging mode. In a transcritical cycle, the main losses are usually due to the difference(s) in heat capacity where the heat exchange occurs, with a variable heat capacity in the working fluid (e.g., the heat capacity of the working fluid varies with temperature at a constant pressure) and a constant heat capacity in the storage fluid. At least some of the systems patented and/or published by ABB compensate with storage fluid flows to and from multiple tanks.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.